Hot-dip Al—Zn alloy-coated steel sheets have both the sacrificial protection of Zn and the high corrosion resistance of Al, and thus rank highly in terms of corrosion resistance among hot-dip galvanized steel sheets. For example, PTL 1 (JP S46-7161 B) discloses a hot-dip Al—Zn alloy-coated steel sheet in which the hot-dip coating contains from 25 mass % to 75 mass % of Al. Due to their excellent corrosion resistance, hot-dip Al—Zn alloy-coated steel sheets have been the subject of increased demand in recent years, particularly in the field of building materials for roofs, walls, and the like that undergo long-term exposure to outdoor environments, and the field of civil engineering and construction for guardrails, wiring, piping, sound proof walls, and the like.
The hot-dip coating of a hot-dip Al—Zn alloy-coated steel sheet includes a main layer and an alloy layer present at an interface of the main layer with a base steel sheet. The main layer is mainly composed of regions where Zn is contained in a supersaturated state and Al is solidified by dendrite solidification (α-Al phase dendritic regions), and remaining interdendritic regions between the dendrites, and has a structure with the α-Al phase stacked in multiple layers in the thickness direction of the hot-dip coating. Due to such characteristic hot-dip coating structure, the corrosion path from the surface becomes complex, making it difficult for corrosion to reach the base steel sheet. Therefore, better corrosion resistance can be achieved with a hot-dip Al—Zn alloy-coated steel sheet than with a hot-dip galvanized steel sheet having the same hot-dip coating thickness.
The inclusion of Mg in a hot-dip Al—Zn alloy coating is a known technique for further improving corrosion resistance.
In one example of a technique relating to a hot-dip Al—Zn alloy-coated steel sheet containing Mg (hot-dip Al—Zn—Mg—Si coated steel sheet), PTL 2 (JP 5020228 B) discloses an Al—Zn—Mg—Si coated steel sheet in which the hot-dip coating contains a Mg-containing Al—Zn—Si alloy. The Al—Zn—Si alloy contains from 45 wt % to 60 wt % of aluminum, from 37 wt % to 46 wt % of zinc, and from 1.2 wt % to 2.3 wt % of silicon, and has a Mg concentration of from 1 wt % to 5 wt %.
Moreover, PTL 3 (JP 5000039 B) discloses a surface treated steel material having an Al alloy coating containing, by mass %, from 2% to 10% of Mg, from 0.01% to 10% of Ca, and from 3% to 15% of Si, the balance being Al and incidental impurities, and having a Mg/Si mass ratio in a specific range.
Hot-dip Al—Zn alloy-coated steel sheets that are to be used in the automotive field, and particularly those that are to be used for outer panels, are typically supplied to automobile manufacturers and the like in a state in which production up to hot-dip coating in a continuous galvanizing line (CGL) has been completed. After being worked into the shape of a panel component, the hot-dip Al—Zn alloy-coated steel sheet is typically subjected to chemical conversion treatment, and also general coating for automobile use by electrodeposition coating, intermediate coating and top coating. However, when a coating film of an outer panel obtained using a hot-dip Al—Zn alloy-coated steel sheet is scarred, the resulting scar acts as a start point for selective corrosion of interdendritic regions present at the interface of the coating film and the hot-dip coating that contain a large amount of Zn. As a result, there have been cases in which significantly greater coating film blistering has occurred than with a hot-dip Zn coating and in which it has not been possible to ensure adequate corrosion resistance (post-coating corrosion resistance). In response, PTL 4 (JP 2002-12959 A), for example, discloses a hot-dip Al—Zn alloy-coated steel sheet in which the formation of red rust from edge surfaces of the steel sheet is improved by adding Mg, Sn, or the like to the hot-dip coating composition in order that a Mg compound such as Mg2Si, MgZn2, Mg2Sn, or the like is formed in the hot-dip coating layer.